Fuel economy indicators are generally known in which the instantaneous value of miles-per-gallon is displayed to the vehicle operator by means of an analog or digital display. The miles-per-gallon value is computed using sensors which provide the instantaneous rates of fuel usage and vehicle speed. Indicators of this type require careful calibration to perform properly, and have failed to achieve widespread acceptance. In addition, a special problem arises in adapting them for use with diesel engines, since the design of diesel engine fuel systems is such that a portion of the fuel flowing to the engine is returned unconsumed to the fuel tank.
In one type of diesel engine fuel system, the ratio between total fuel flow and fuel consumption varies continuously. In such a system, there is no single point where a flow sensor can be placed to measure the fuel consumed. Two methods have been devised to provide fuel economy indicators for such systems. The first involves the use of two flow sensors, one to measure the total flow from the tank to the engine, and the second to measure the return flow from the engine to the tank, with the fuel consumed being calculated electronically by subtraction. To achieve accurate flow measurements using two sensors, it is necessary either to use two sensors closely matched for absolute accuracy, or to provide means for individual calibration of each flow sensor to compensate for their accuracy differences. Either approach adds substantially to the cost of the instrument. To achieve high levels of absolute accuracy using volumetric flow sensors, it is also necessary to measure the return flow temperature and compensate electronically for the effect of its higher temperature upon the specific gravity and viscosity of the fuel.
A second technique that can be used for variable ratio diesel fuel systems is to modify the system by adding an intermediate return flow tank which recycles the return flow directly to the engine. In this system, the flow from the main fuel tank to the engine equals the fuel consumed and this value